1. Field of the Invention
The present invention relates to a method for growing single crystals of perovskite oxides, more particularly, to a method for growing single crystals of perovskite oxides characterized in that a perovskite seed single crystal such as barium titanate (BaTiO3) etc. is adjoined to a polycrystal of perovskite oxides such as barium titanate (BaTiO3) etc., which is controlled in its composition ratio and/or in the added amount of its specific components, and then the adjoined combination is heated, herein, said heat treatment and said control of the composition ratio and the added amount are performed so as to make the seed single crystal grow into the polycrystal at the interface between the seed single crystal and the polycrystal(adjoined part) and to repress secondary abnormal grain gowths inside the polycrystal.
In addition, the invention relates to a method for producing single crystals of perovskite oxides at low costs on a large scale by using the single crystal, which has the composition of the polycrystal, produced according to the above method as a seed single crystal.
In the present invention, “primary abnormal grain growth” means the abnormal grain growth occurring in advance during a heat treatment under the Eutectic temperature, which is known as a conventional and usual “abnormal grain growth”.
In the present invention, “secondary abnormal grain growth”, which is different from the primary abnormal grain growth, means the abnormal grain growth that occurs again during a long-term heat treatment over a specific temperature after the completion of the primary abnormal grain growth, wherein a bimodal distribution of grain sizes is disappeared and a distribution of grain sizes comes to be uniform.
In the present invention, “combination” means a polycrystal and a seed single crystal adjoined to the polycrystal.
2. Description of the Related Art
The “perovskite oxides” as used herein have a chemical formula of “ABO3”, e.g., BaTiO3. In Pb-type perovskite oxides, Pb substitutes for entire or a portion of “A” of the above formula, e.g., “(PbxA1−x)BO3” (0 x 1) of a simple form or “(PbxA1−x) (ByC1−y)O3”(0 x 1; 0 y 1), in which the number of the atoms substituting for “A” or “B” increases. Pb-type perovskite oxides include PbTiO3(PT), (Pb, Ba)TiO3, Pb(ZrxTi1-x)O3(PZT), Pb(Mg1/3Nb2/3)O3 (PMN), (1−x) PMN-xPT, (1−x−y)PMN-xPT-yPZ, Pb(Zn1/3Nb2/3)O3 (PZN) or (1−x) PZN-xPT, (1−x−y)PZN-xPT-yPZ, etc.
The single crystals of perovskite oxides are widely applied in various fields, including optical, piezoelectric, electric or mechanical field, etc., and the application fields will be extended with industry development.
The single crystals of undoped barium titanate and barium titanate solid solution are widely used as a material for piezoelectric devices and optical devices such as optical valve, optical interrupter, and phase-matching mirror, etc. and considered as a promising substrate material for various thin film elements.
In Pb-type perovskite oxides, particularly, the single crystals of Pb(ZrxTi1−x)O3(PZT), (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3(PMN-PT) or (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3(PZN-PT), etc. and the solid solution thereof are considered as promising materials for electronic devices, because of their high dielectric and excellent piezoelectric properties such as remarkable electro-mechanical coupling factors.
The conventional methods for growing a single crystal of barium titanate (BaTiO3), barium titanate solid solution, Pb-type perovskite and Pb-type perovskite solid solution require expensive facilities, however, involves many problems in producing a large amount of big single crystals because of extremely complicated process for growing the single crystals and have difficulty in the application because of the high expense.
In particular, Pb-type perovskite oxides have serious problems because lead oxide(PbO) having a strong volatility volatilizes when single crystals grow. Further, the conventional methods for growing a single crystal of Pb-type perovskite oxides and the solid solution thereof necessarily require a melting process, and thus make the entire composition change and the phase of the perovskite unstable owing to the volatilization of PbO. Therefore, it is difficult to produce a single crystal having a specific size intented as its final size and property. In addition, it is difficult to produce in large quantities because of the difficulty in the production processes and the requirement of expensive facilities.
Since the emergence of Flux method for single crystal growth of (1−x) Pb (Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT), the subsequent methods for single crystal growth have been developed such as the Bridgman Method, etc. However, these general methods such as Flux Method or Bridgman Method using a melting process, etc. present some problems in the production of PMN-PT single crystal in that it is difficult to maintain the uniform composition of the growing single crystal owing to the volatilization of PbO during the melting process. Therefore, the processes require complicated facilities and skilled functions and are difficult to produce single crystals in large quantities at low costs.
The production of single crystals of Pb(ZrxT1−x)O3(PZT) having an actually applicable size by general liquid-state single crystal growth methods is considered to be impossible, because of the difficulty in repression of the strong volatilization of PbO and the separation to liquid phase and ZrO2 during melting, i.e., Incongruent Melting. In case of the mass production of single crystals of PZT, which is one of materials having the most excellent piezoelectric property, the produced PZT can substitute for the conventional piezoelectric polycrystal and materials for single crystals in various application fields.
Grain growth takes place during the step of sintering polycrystals, in which case only a few grains are sometimes rapidly grown in an abnormal manner relative to the most normal grains.
It is appreciated that controlling the growth of such a few abnormal grains in polycrystals may allow single crystal to be easily produced without a melting process.
A general method for single crystal growth using the melting process is called Liquid-state Single Crystal Growth (LSCG) method, and a method for single crystal growth by heat treatment of polycrystals is referred to as Solid-state Single Crystal Growth (SSCG) method. The SSCG method has been suggested since the 1950's and demonstrated as an effective method of preparing single crystals of a metal, which is limited to only a few types. It is however reported that the method has a difficulty in preparing single crystals large enough for practical use from an oxide, because the growth is too slow in grain growth and hard of controlling nucleation of abnormal grains.
Since the emergence of the Flux method for single crystal growth of barium titanate, the subsequent methods for single crystal growth of barium titanate have been developed such as Zone Melting method and the Top-Seeded Solution Growth (TSSG) method. The single crystals of barium titanate grown by the Flux method have a thickness of less than 1 mm and a diameter of several millimeters and thus actually restrained in practical uses. It is known that the TSSG method, which has the advantages of the Flux method and the Czochralski method, is applicable to the growth of relatively large single crystals of barium titanate almost without residual stress. However, the TSSG method also requires complicated facilities and skilled functions and is inadequate as a method for preparing a large amount of single crystals at low costs.
Meanwhile, there has been made an attempt to obtain single crystals by subjecting polycrystals of ferrite, barium titanate [BaTiO3], aluminum oxide [Al2O3] and PMN-PT to heat treatment through Solid-State Single Crystal Growth(SSCG) Method. This method for single crystal growth involves sintering a powder impregnated with single crystals as seed single crystals or providing an interface between the polycrystals and the seed single crystals, followed by heat treatment.
Disadvantageously, the method is not suitable to preparing single crystals large enough for practical uses such as more than several mm because the growth of single crystals is retarded relative to the conventional Liquid-State Single Crystal Growth methods.
Even though single crystals are produced by using abnormal grain growth phenomenon occurring in the polycrystal, it is difficult to continue to grow single crystals because the abnormal grains of the polycrystal repress the growth of the seed single crystals when the growing seed single crystals meet peripheral abnormal grains. Therefore, the conventional Solid-State Single Crystal Growth(SSCG) method is less advantageous than the conventional Liquid-State Single Crystal Growth method, in that it is difficult to produce single crystals having an actually applicable large size and the reproduction possibility is low because it is impossible to control the abnormal grain growths occurring inside the polycrystal by the method. In particular, in the case of PMN-PT, it is difficult to produce single crystals having a size of more than several mm because of the trouble in the control of abnormal grain growths in the polycrystal.
For single crystal growth of barium titanate [BaTiO3], there is reported a method for preparing single crystals by adding particles having a (111) double twin plate or a seed forming agent to form a (111) double twin plate.
However, this method also has problems that it cannot produce single crystals without a (111) double twin plate and cannot produce in large quantities single crystals large enough for practical use at low costs because it is difficult to control secondary abnormal grain growth, create a single crystal and continue to grow only the single crystal.